Spark Plug Insulator Seal Design for Airtightness
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Solution Overview
Problem
Spark plugs with reduced diameter face challenges in maintaining mechanical strength and seal performance due to insufficient or excessive deformation of the metallic shell, leading to potential damage and changes in thermal characteristics.
Innovation Solution
A spark plug design featuring a rodlike center electrode, an insulator with specific diameter-reducing portions, and a metallic shell with a protrusion, where an annular seal member is positioned to apply an unbalanced load on the metallic shell diameter-reducing portion, enhancing seal performance and restraining deformation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If the diameter of the spark plug is reduced to improve design freedom, then the mounting hole diameter can be reduced, but the mechanical strength of the insulator deteriorates
Solution Approach 1:
The insulator is divided into multiple diameter sections (first diameter, second diameter smaller than first, third diameter smaller than second), creating stepped portions that distribute mechanical loads and prevent concentration of stress at any single location, thereby maintaining strength despite overall size reduction
Solution Approach 2:
The insulator features localized diameter reductions at specific positions rather than uniform thinning, allowing the insulator to maintain adequate wall thickness and mechanical strength in critical areas while reducing diameter in non-critical areas to achieve compact overall dimensions
2Reliability
If the metallic shell is deformed to provide seal between insulator and shell, then seal performance is improved, but the insulator may be damaged due to excessive deformation
Solution Approach 1:
A packing (seal member) is introduced as an intermediary element between the metallic shell and insulator, allowing the shell to be deformed for sealing without the deformation force being directly transmitted to and damaging the insulator
Solution Approach 2:
The packing is positioned beforehand between the shell and insulator to cushion and absorb the deformation forces during assembly, preventing excessive stress from reaching the insulator while still enabling adequate seal formation
3Reliability
If the metallic shell is excessively deformed to ensure seal, then seal performance is improved, but the insulator protruding dimension changes affecting thermal characteristics
Solution Approach 1:
The packing acts as a compliant intermediary that absorbs variations in shell deformation, maintaining consistent positioning of the insulator relative to the shell regardless of manufacturing tolerances or assembly variations, thereby preserving thermal characteristics
Solution Approach 2:
The use of a deformable packing material allows the sealing interface to accommodate parameter variations in shell deformation while maintaining functional performance, decoupling the seal quality from precise dimensional control of the rigid components
Data Source
Figure 1
Figure 2(A)~3(C)
Figure 4~5(C)
AI summary
[Objective] To favorably provide airtightness between a metallic shell and an insulator. [Means for Solution] A spark plug includes a center electrode, an insulator, a metallic shell, and a seal member for providing a seal between the insulator and the metallic shell. The insulator includes a first portion, a second portion located axially forward of the first portion and being smaller in outside diameter than the first portion, and an insulator first-diameter-reducing-portion whose outside diameter reduces forward and which connects the first portion and the second portion. The metallic shell includes a protrusion protruding radially inward, and the protrusion includes a metallic shell diameter-reducing-portion whose inside diameter reduces forward. The seal member is disposed between the insulator first-diameter-reducing-portion and the metallic shell diameter-reducing-portion at such a position as to cross an extension line formed by imaginarily extending forward the outer surface of the first portion. An angle θ21 and an angle θ22 satisfy θ21 > θ22, where, in a section which contains an axial line, the angle θ21 is an angle between a straight line orthogonal to the axial line and the outline of the metallic shell diameter-reducing-portion, and the angle θ22 is an angle between the straight line and an outline of the insulator first-diameter-reducing-portion.